1. Field of the Invention
This invention relates to electrical safety equipment and is specifically directed to an apparatus and a method for preventing electrical arcing between two or more machines receiving power through lengthy power cables from a common source. The invention is of particular importance in the suppression of the dangerous arcing which may occur when two or more electrical machines supplied from a common power source come into contact and then separate, as in the case of mining machinery operated in volatile, explosive atmospheres.
2. Description of the Prior Art
In numerous applications, power for electrical machinery is supplied through lengthy multi-conductor cables. Mining equipment, for example, may require cables from several hundred to several thousand feet long. Such lengthy cables are obviously highly susceptible to damage, giving rise to substantial shock and arcing hazards.
The first type of hazard is the possibility of short circuits occurring between two or more power conductors or between a power conductor and a ground conductor. One cause of such short circuits is damage to the power cable. Another cause is equipment failure, such as insulation breakdown and current leaks. A short circuit may result in dangerous voltage being placed on conductive external housings of electrically powered machines, such as the conductive frame of a mining car, which is hazardous both to workers and equipment.
The second type of hazard is caused by small differences in the unbalanced voltages which may be induced in the grounding conductors of two power cables when they are connected under load conditions. Where, as is frequently the case, the grounding conductor in the power cable is not symmetrically positioned with respect to the power conductors, such as in some three-phase alternating current systems (as are usually used for high-power equipment), a voltage may be induced in the ground conductors. Since the ground conductors of any two such machines are connected to the same reference earth ground, the unbalanced induction of voltage in the ground conductors may result in there being several volts (e.g., up to 6 or 7 volts) difference in potential between the conductive external equipment housings (e.g., supporting vehicles, frames, bases, enclosures, etc.) to which the ground conductors are attached. Since low impedance circuits are involved, even a small voltage can cause large currents in the ground conductors should the equipment housings come into contact. Then, upon separation of the housings, arcing will occur. In a possibly explosive environment, such as in the atmosphere of a coal mine, this arcing presents a severe hazard.
Connecting a grounding conductor between the housing of the involved mining equipment and reference earth ground reduces the shock hazard due to short circuits. By monitoring current in the grounding conductor, short circuits may be automatically detected; once this condition is detected, power may be automatically disconnected from the cable. Usually, power for motors or other machinery is provided by a three-phase high-voltage source; then, a Y-connected power supply transformer secondary can be used to establish a common reference point at the junction of the transformer windings. Current in the grounding conductor will then be solely due to short circuits to the grounding conductor, stray fields, or two machines coming into contact.
Prevention of the hazard due to arcing is the object of a regulation promulgated by the U.S. Bureau of Mines, 30 C.F.R. Para. 75.524. The regulation places a limit on permissible arcing current level so that arc energy is maintained at an amount considered safe by the Bureau. The arcs proscribed are those which carry sufficient energy to ignite atmospheres typically encountered in mine environments. Under this standard, when two machine grounds are connected through a 0.1 ohm resistor of sufficient power handling capacity, no more than 1.0 amperes may flow between the machines.
Two systems have heretofore been proposed to alleviate the arcing problem.
The first system, proposed by the Bureau of Mines, was to incorporate, between the cable grounding conductors and the reference earth ground, an energy absorbing circuit comprising a full-wave diode bridge. With the proper number of diodes in each leg, the bridge would provide a high impedance path for applied A.C. voltages of about 7 volts or less, and a low impedance path for the higher voltages present when a power conductor short circuits to the cable grounding conductor or machine housing.
The second system, described in U.S. Pat. No. 3,855,501, involves the use of a saturable core reactor connected in series with the power grounding conductor. Under the modest voltages typically induced in a cable grounding conductor, the reactor's core is unsaturated, and a considerable impedance is present between the grounding conductor and reference earth ground, thereby limiting current. When a short circuit to the grounding conductor or machine housing occurs, however, the reactor core saturates and the reactor presents only a very small impedance between the cable grounding conductor and reference earth ground.
Both the diode bridge and the saturating core reactor introduce some impedance in series with the cable grounding conductor, under short circuit conditions. This is undesirable because the impedance may limit the current carrying capacity of the ground path and result in a dangerous voltage appearing on the machine housing. Also, the impedance element may fail as a result of the excessive power dissipation required under sustained short circuit conditions. In the case of the saturable core reactor, voltage spikes will appear on the machine frame during ground fault current flow. These voltage spikes occur during the brief interval at the beginning of each half cycle of current flow when the reactor core has not yet reached saturation. Although these spikes have low energy content, workmen are endangered by them since the housing of the associated machine is electrically "hot" with respect to earth at these times. The diode bridge system is not subject to this drawback, but it has other problems such as the fact that under short circuit conditions, high current levels may damage one or more diodes. In the most common failure modes this will result in the short circuiting of the damaged diode(s). Thus, once the power circuit short is relieved, there will certainly be less arc prevention capability, with it being quite likely there will be no arc prevention capability.